Cache behaviour of QLibrary

If a library tries to load a plugin, but the load fails (in load_sys()) because of missing dependencies, the cache entry is done in any case. When solving the missing dependencies, the plugin isn't tested again, because of the cache entry. So, in case of an error, no cache entry should be made in the registry.

As a practical example:

• An incorrect setup is delivered to the customer with one DLL missing on which a plugin depends.
• The application fails to load the plugin, and a registry cache entry is done.
• A corrected setup is sent to the customer, which conatains the missing DLL.
• The application is still not working, because of the cache entry...

Anyway, for the Debug version of the library, no caching should be done.